1. Field of the Invention
The present invention relates to a television tuner.
2. Description of the Related Art
A conventional tuner used for a television receiver and the like will be described with reference to FIG. 4. A high frequency tuning circuit 11 is connected between an antenna 10 and a mixer 1 of a first frequency converting circuit 6, a first tuning voltage VT1 of a variable voltage source 9 is supplied as a tuning voltage of a variable capacity diode, which forms a first local oscillating circuit 2, and a second tuning voltage VT2, which is obtained by expanding the variable range of the first tuning voltage VT1, is supplied as the tuning voltage of the variable capacity diode, which forms the high frequency tuning circuit 11. The high frequency tuning circuit 11 and the first local oscillating circuit 2 include variable capacity diodes as a capacitive element respectively.
The second tuning voltage VT2 can be obtained by applying the first tuning voltage VT1 to an expander 12, and the second tuning voltage VT2 has a variable range, which is expanded 7 times larger than that of the first tuning voltage VT1. The second tuning voltage VT2 is made to have the 7 times larger variable range than that of the first tuning voltage VT1 on the assumption that it will satisfy the condition, on which tracking errors do not occur, if the variable ratio of the total capacitance of the high frequency tuning circuit 11 becomes 1.21 by making the variable range of the second tuning voltage VT2 7 times, which is close to 7.34, larger than that of the first tuning VT1, since the variable ratio of the total capacitance of the high frequency tuning circuit 11 is 7.34 times larger than that of the first local oscillating circuit 2.
Also, a received high frequency signal is mixed with the oscillating signal of the first local oscillating circuit 2, and then becomes a first intermediate frequency signal. The first intermediate frequency signal is amplified in a first intermediate frequency amplifying circuit 3, and then mixed with the oscillating signal of a second frequency converting circuit 5 so as to become a second intermediate frequency signal. The second intermediate frequency signal is applied to a detection circuit (not shown) through a terminal 8 (For example, see Japanese Unexamined Patent Application Publication No. 07-283693 (FIG. 1).
In the case of a variable capacity diode used for the high frequency tuning circuit 11 and the first local oscillating circuit 2, an anode is grounded, and a cathode is applied with the tuning voltage, but tracking error does not occur. Also, when the variable capacity diode having the above structure receives a television signal, the tuning characteristic of the high frequency tuning circuit 11 is set to have the video carrier frequency P at the peak and the voice carrier frequency S at a location several dB lower than that of the video carrier frequency P as shown in FIG. 5A, thereby the signal is frequency-converted to the intermediate frequency signal. It is because the frequency characteristic of the video signal after demodulation is required to be flat.
However, if the receiving band is widened to receive the FM broadcasting, and the FM broadcasting is frequency-converted into the television voice intermediate frequency signal at the above state, the tuning characteristic of the high frequency tuning circuit is changed to have a frequency P′, which is 4.5 MHz lower than the frequency F of the FM broadcasting signal, which is to be received, at the peak as shown in FIG. 5B, and there is a problem in that the level of the FM broadcasting signal, which is to be received, is lowered, thereby the receiving sensitivity deteriorates.